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Abstract

Gorgonian corals occur extensively at continental slope depths > 200 m off the southwest
Grand Banks of Newfoundland. Among these corals, Keratoisis grayi forms
gorgonian coral thickets on cobbles and boulders in otherwise muddy sand habitats.
These thickets are believed to form a critical benthic habitat, in particular for juvenile
fish, and as such are an integral part of the ecosystem. These coral thickets are
impacted by bottom trawling activity which therefore could have far reaching consequences
for the larger ecosystem. This thesis reports on a study of how the ocean
bottom boundary layer is affected by the presence of coral thickets. This information
is important both to establish the characteristics of coral habitat but also to demonstrate
how the removal of corals modifies the boundary layer which would in turn
modify the benthic environment.
Bottom boundary layer currents in coral habitat in Haddock Channel were characterized
using two 2-MHz acoustic Doppler current profilers.The profilers were deployed
on the seafloor at a depth of 700 m, looking upward, for 85 hours, beginning
July 17th, 2007. The effective vertical profiling range was 4 meters, with 1 meter
depth resolution, sampling every 2.7 minutes. One instrument was placed in an area
where bamboo corals (Keratoisis grayi ) extend approximately to 1 meter in height
and occur with a density on the order of 1 colony per square meter (Coral Site). The
second instrument was deployed 100 meters away in an area with visually similar sea floor characteristics, but from which the corals had been removed by a research
bottom trawl (Mud Site). Mean flow speeds at both the Mud and Coral Site are
on the order of 10 cm s⁻¹, which is consistent with previous current data from the
general area. Observed currents showed some evidence of tidal forcing but other nonlinear
processes clearly influence the current regime. Speed profiles were fitted to the
logarithmic law of the wall to obtain bottom roughness zₒ, and friction velocity u∗
estimates. Both the Mud and Coral Site appear to conform to the logarithmic law of
the wall for turbulent boundary layers.
Friction velocity (u∗) estimates at flow speeds less than 5 cm s⁻¹, were consistently
higher at the Coral Site, relative to the Mud Site; Mud Site u∗ values were 30%
to 80% of Coral Site estimates, indicating increased turbulence due to the presence
of corals. However, friction velocity increased faster with flow speed at the Mud
Site, suggesting that at higher flow speeds coral induced bottom roughness is less
important to friction velocity. There was significant uncertainty in bed roughness
estimates, however bed roughness values at the Coral Site (mean zₒ = 0.51 ± 0.28
cm), were found to be generally higher at flow speeds below 5 cm s⁻¹, compared to
the Mud Site (mean zₒ = 0.27 ± 0.40 cm), again possibly indicating that the coral
are affecting the hydrodynamic roughness at low speeds.
Backscatter intensity was also examined as an indication of suspended organic
material. It was impossible to make relative absolute comparisons between the two
sites but relative changes in backscatter intensity could be compared. Backscatter
levels from both sites increased as flow speeds increased, up to 7 cm s⁻¹. Backscatter
increased faster with flow speed at the Mud Site, relative to the Coral Site, for speeds
between 2.5 and 7 cm s⁻¹, which is broadly consistent with the suggestion of greater
increase in friction velocity values seen at the Mud Site.
Above 7 cm s⁻¹, Coral Site backscatter intensity increased substantially, while the corresponding Mud Site backscatter intensity declined. The reason for this change in
trend is unclear, a possible explanation being a transition into a different flow regime
whereby increased flow speeds interact favourably with the rough topography created
by the corals. However, the large uncertainty estimates for backscatter levels at these
higher speeds were such that it was very difficult to draw firm conclusions.
We conclude that higher u∗ and zₒ estimates at low flow speeds at the Coral Site,
relative to the Mud Site, are consistent with the hypothesis that the enhancement of
turbulence due to the coral behaving as roughness elements is significant only at low
flow speeds, enabling coral polyps greater opportunities to extract organic material
from the water column due to resuspension of organic material from the sea floor.